1. Field of the Invention
The invention relates to composite structural articles, and more specifically to composite articles which are prepared from structural fibers or stranded fibers sewn together by means of meltable resin holding thread and subsequently infused with a curable resin.
2. Discussion of the Background
Increasing demands in the transportation industry, including the rail, marine and aerospace fields, coupled with the increasing cost of energy, has placed a high demand on structural materials that are of extreme strength, durability and at the same time, lightweight. In particular, aerospace applications require a very high standard of safety and performance. These high standards place extremely critical demands on the structural materials used in aerospace applications and require a level of performance over and above standard lightweight metal alloys.
Thus, replacements for conventional alloys and even lighter weight alloys such as aluminum-lithium are constantly being sought. One such group pf substitutes includes the composites field, generally including fiber reinforced plastics. One important type of produce within this field is the structural article made by infusing or impregnating a stitched fiber form comprised of a plurality of plies of knit fabric made of structural fiber with a curable resin and thereafter molding the product to provide a stiff, lightweight product which is very cost effective to manufacture. Typical structural fibers may be prepared from fiberglass, Kevlar.RTM., boron and carbon. U.S. Pat. Nos. 4,681,049; 4,664,961; 4,567,738 and 4,484,459, among others disclose examples of such fiber forms and composite products.
The various plies of the knitted fabric comprised of structural fiber are assembled and may be stitched together. This assembly (fiber form) is then generally molded or shaped into the approximate shape of the structural article desired, then stitched in one or more directions to secure the fiber form in the desired oriented position. This "perform" is then infused or impregnated with a resin which is subsequently cured to yield the composite article.
The yarns used to knit the structural fibers together into fabrics and the fabric assembly stitching threads (generically, holding means) are generally polyester or polyamide threads such as Kevlar.RTM.. These fabric knitting yarns and stitching threads remain in position throughout the resin impregnation step and can also be found in the finished composite article.
Such knitted fabric fiber forms molded into composite articles offer the industry a highly cost effective manufacturing process. The knitting and stitching process results in a distribution of fibers in the composite which is different from undirectional prepreg tape composites. Such distribution leads to differences in mechanical properties of the composite articles. Knitted fabric composites are generally much stronger and stiffer than woven material composites and somewhat less strong in some aspects compared to undirectional tape composites.
When knitting or stitching with conventional holding means such as polyester or polyamide thread, the uniform distribution of the structural fibers is disturbed giving rise to resin rich areas. These resin rich areas may limit the full translation of the constituent material properties into the composite material. As the fabrics are assembled with secondary knitting and assembly operations, the structural fibers are deformed slightly as knitting yarn or other holding means passes around or through them, reducing the uniformity of distribution of fibers within each ply. Subsequent stitching of knitted fabric plies often involves further penetration of individual tows in the plies, thus causing a local distortion or waviness from the desired alignment or orientation. This waviness of the fibers may lead to the earlier onset of compression strength failure of these composite materials relative to materials with straight fibers. The deformed microstructure may also be detrimental to other matrix dominated composite mechanical properties such as shear strength.
Further, some conventional knitting yarns and stitching threads have low adhesion potentials for certain resins which surround all the fibers in the composite to form chemical and mechanical bonds. This low adhesion with some resins may result in knit yarn or stitch thread disbonding and may contribute to the formation of microcracks within the composite under thermal loads.
The conventional knitting yarns or stitching threads also generally have different thermal and mechanical properties relative to the resin. For composites made from knit fabrics the use of conventional knitting yarns and stitching threads may produce non-uniform structural fiber distributions and local distortions in the orientation of the structural fibers giving rise to stress points under thermal and mechanical loading. This may contribute to the disbonding or the formation of microcracks as noted above.
Conventional fiber reinforced composite materials, even those having resin rich spaces within the fiber network as described above, have satisfactory performance characteristics for most marine and aerospace applications. However, the aerospace field is placing ever increasing demands on the performance characteristics of structural materials intended for aerospace applications. This is particularly true in the field of military aerospace applications where levels of performance over and above that of conventional composite materials are required. The increasingly severe conditions encountered in military aerospace applications require increasingly severe safety and performance standards.
Accordingly, a need exists for a method of preparing fiber reinforced composite materials with performance characteristics which exceed those of conventional composite materials at a reasonable cost. New composite materials and methods of preparing them contribute greatly to technical advances in aerospace applications.